1. Field of the Invention
The present invention relates to an exhaust gas purifier for an internal combustion engine.
2. Description of the Prior Art
FIG. 19 is a structural diagram of an exhaust gas purifier for an internal combustion engine 1 disclosed by Japanese Laid-open Patent Application No. 8-232644. In FIG. 19, reference numeral 1 denotes an internal combustion engine, 2 a piston, 3 a combustion chamber, 4 a spark plug, 5 an inlet valve, 6 an inlet port, 7 an exhaust valve, and 8 an exhaust port. The inlet port 6 is connected to a surge tank 10 via a branch pipe 9, and a fuel injection valve 11 for injecting fuel into the inlet port 6 is disposed in the branch pipe 9. The surge tank 10 is connected to an air cleaner 13 via an inlet duct 12, and a throttle valve 14 is arranged in the inlet duct 12. The exhaust port 8 is connected to a casing 17 incorporating an NOx absorber catalyst 18 via an exhaust manifold 15 and an exhaust pipe 16.
An electronic control unit 30 is a digital computer which comprises a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, a back-up RAM 35 which is always connected to a power source, an input port 36, an output port 37, AD converters 38 and drive circuits 39 all of which are connected to one another by a bilateral bus 31. A pressure sensor 19 for outputting an output voltage proportional to the absolute pressure of the surge tank 10 is placed in the surge tank 10, and the output voltage of the pressure sensor 19 is applied to the input port 36 through the corresponding AD converter 38. A switching type air-fuel ratio sensor 20 called xe2x80x9cO2 sensorxe2x80x9d is installed in the exhaust manifold 15 at an upstream of the NOx absorber catalyst 18, and the output of the air-fuel ratio sensor 20 is applied to the input port 36 through the corresponding AD converter 38. Another switching type air-fuel ratio sensor 22 called xe2x80x9cO2 sensorxe2x80x9d is installed in the exhaust pipe 21 at a downstream of the NOx absorber catalyst 18, and the output of the air-fuel ratio sensor 22 is applied to the input port 36 through the corresponding AD converter 38. A revolution sensor 23 for generating an output pulse indicative of an engine speed N and a car speed sensor 24 for generating an output pulse indicative of a car speed are connected to the input port 36. The output port 37 is connected to the spark plug 4 and the fuel injection valve 11 via the respective drive circuits 39.
A description is subsequently given of the operation of the prior art device. A mixture having a lean air-fuel ratio (to be referred to as xe2x80x9clean mixturexe2x80x9d hereinafter) is generally burnt in a combustion chamber 3. When the amount of nitrogen oxide NOx absorbed by the NOx absorber catalyst 18 exceeds a predetermined amount, the NOx discharge function of the NOx absorber catalyst 18 is started by controlling such that the air-fuel ratio of exhaust gas flowing into the NOx absorber catalyst 18 becomes rich.
However, the deterioration of the NOx absorber catalyst is roughly divided into two types: (1) deterioration such as thermal deterioration by which the catalyst cannot restore its NOx absorbing capability and (2) deterioration such as sulfur poisoning from which the NOx absorber catalyst can restore its NOx absorbing capability to a certain extent by some regeneration treatment. In the exhaust gas purifier for an internal combustion engine 1 disclosed by the aforementioned Japanese Laid-open Patent Application No. 8-232644, the above two different types of deterioration are handled in the same manner and the permissible amount of absorbed NOx is set based on the NOx absorbing capacity after judgment on deterioration. Since rich operation for judging the deterioration of the NOx absorber catalyst is carried out based on the air-fuel ratio A/F set as one pattern in the above publication, as shown in FIG. 20, when the air-fuel ratio for judgment is rich, a difference in deterioration is small whereas, as shown in FIG. 21, when the air-fuel ratio is not so rich, the difference becomes large but rich operation for judgment lasts for a long time, which is disadvantageous from the view point of fuel cost.
The NOx absorber catalyst 18 deteriorates gradually while it is used. The NOx absorbing capability of the NOx absorber catalyst 18 degrades as it deteriorates and the catalyst cannot absorb NOx in the end. Therefore, when the NOx absorber catalyst 18 is used, it is necessary to detect how much the NOx absorber catalyst 18 deteriorates. When the air-fuel ratio of exhaust gas flowing into the NOx absorber catalyst 18 is made rich, the air-fuel ratio of exhaust gas flowing out from the NOx absorber catalyst 18 becomes slightly lean while the NOx discharge function of the NOx absorber catalyst 18 is executed. When the NOx discharge function of the NOx absorber catalyst 18 is completed, it is found that the air-fuel ratio of exhaust gas flowing out from the NOx absorber catalyst 18 becomes rich. In this case, as the amount of NOx absorbed by the NOx absorber catalyst 18 is smaller, a time elapsed from the time when the air-fuel ratio of exhaust gas flowing into the NOx absorber catalyst 18 is made rich to the time when the air-fuel ratio of the exhaust gas flowing out from the NOx absorber catalyst 18 becomes rich is shorter. Therefore, the deterioration degree of the NOx absorber catalyst 18 can be detected from the above time elapsed.
It is an object of the present invention which has been made to solve the above problems to provide an exhaust gas purifier for an internal combustion engine which is capable of detecting the deterioration degree of an NOx absorber catalyst making use of a time elapsed from the time when the air-fuel ratio of exhaust gas flowing into the NOx absorber catalyst is made rich to the time when the air-fuel ratio of exhaust gas flowing out from the NOx absorber catalyst becomes rich.
According to a first aspect of the present invention, there is provided an exhaust gas purifier for an internal combustion engine which comprises an NOx absorber catalyst, air-fuel ratio sensor, first NOx estimating means, decision means, air-fuel switching means, second NOx estimating means and deterioration judging means, wherein the NOx absorber catalyst absorbs NOx when the air-fuel ratio of inflow exhaust gas is lean and purifies and discharges its absorbed NOx when the air-fuel ratio of inflow exhaust gas is rich, the air-fuel ratio sensor detects the concentration of oxygen contained in exhaust gas at a downstream of the NOx absorber catalyst, the first NOx estimating means obtains a first estimated amount of NOx estimated to be absorbed by the NOx absorber catalyst, the judging means judges whether the first NOx estimated amount obtained by the first NOx estimating means exceeds a decision level for judging that the first NOx estimated amount becomes the NOx maximum permissible amount of the NOx absorber catalyst, the air-fuel ratio switching means switches the air-fuel ratio of the exhaust gas from lean to rich to discharge NOx from the NOx absorber catalyst when the judging means judges that the first NOx estimated amount exceeds the above decision level, the second NOx estimating means obtains a second estimated amount of NOx from the length of a time elapsed from the time when the air-fuel ratio switching means switches the air-fuel ratio of exhaust gas from lean to rich to the time when the output of the air-fuel ratio sensor is changed to an output level corresponding to a rich air-fuel ratio, and the deterioration judging means judges the deterioration degree of the NOx absorber catalyst by comparing the second estimated amount of NOx obtained by the second NOx estimating means with the first estimated amount of NOx obtained by the first NOx estimating means. The deterioration judging means judges the deterioration degree of the NOx absorber catalyst with accuracy by lengthening a time required for the output of the air-fuel ratio sensor to change from lean to rich by reducing the rich degree of the exhaust gas at the time of switching the air-fuel ratio of the exhaust gas from lean to rich when the time required for the output of the air-fuel ratio sensor to change from lean to rich is shorter than a predetermined value and it is difficult to judge the deterioration degree of the NOx absorber catalyst.
According to a second aspect of the present invention, there is provided an exhaust gas purifier for an internal combustion engine, wherein the purifier further comprises an air-fuel ratio sensor for detecting the concentration of oxygen contained in exhaust gas at an upstream of the NOx absorber catalyst and the second NOx estimating means comprises means of starting the measurement of time when the air-fuel ratio is changed from lean to rich or when the output of the air-fuel ratio sensor at an upstream of the NOx absorber catalyst changes from lean to rich.
According to a third aspect of the present invention, there is provided an exhaust gas purifier for an internal combustion engine which further comprises sulfur regenerating means for carrying out the control of sulfur regeneration when the deterioration judging means judges that the deterioration degree of the NOx absorber catalyst exceeds a predetermined level.
According to a fourth aspect of the present invention, there is provided an exhaust gas purifier for an internal combustion engine which further comprises alarm generating means for generating an alarm when the deterioration judging means judges that the deterioration degree of the NOx absorber catalyst exceeds a predetermined level.
The above and other objects, features and advantages of the invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.